1. Field of the Invention
The present invention relates to a grid-connected inverter that converts DC electric power, generated by solar cells or fuel cells, to AC electric power and is interconnected to a commercial power system.
2. Related Art
Previously, a grid-connected inverter has been provided that converts DC electric power, generated by a solar cell or a fuel cell, to AC electric power and is interconnected to a commercial power system for supplying electric power to loads.
In the following, with reference to FIG. 6 and FIG. 7, a grid connection system including a related interconnection unit will be explained. FIG. 6 is a block diagram showing a solar cell power generation system as a first example of a grid connection system including a related interconnection unit disclosed in JP-A-2008-35655 in which a capacitor current is detected. The power generation system is formed of a solar cell 1 whose generated power varies depending on an amount of solar radiation and a grid-connected inverter 2 converting DC electric power, generated by the solar cell 1, to AC electric power, and is interconnected to a commercial power system 3. The grid connection system supplies AC electric power to a load (not shown) connected between the grid-connected inverter 2 and the commercial power system 3 by switching the operation of the grid connection system to either an interconnection operation of the grid-connected inverter 2 and the commercial power system 3 or an individual operation of only the grid-connected inverter 2.
The grid-connected inverter 2 is formed of a boost chopper 4 stepping up the DC electric power outputted by the solar cell 1, an inverter circuit 5 converting the output voltage of the boost chopper 4 to an AC voltage, a filter circuit 6 smoothing the output voltage of the inverter circuit 5 to make the output voltage have a sinusoidal waveform, a contactor 7 connected between the filter circuit 6 and the commercial power system 3 and a control circuit 8 giving turning-on and -off signals to each of switching devices 51 to 54. In the boost chopper 4, the output voltage of the solar cell 1 varying within the range on the order of 100V to 300V is stepped up to the order of 350V. With the stepped up DC voltage taken as an input voltage, PWM control is carried out in the inverter circuit 5 to convert the DC voltage to an AC voltage. The filter circuit 6 eliminates ripple components with a PWM carrier frequency included in the output of the inverter circuit 5, by which the inputted AC voltage is converted to a sinusoidal AC voltage of the order of 200V (for example, 202V±20V) to be outputted.
The control circuit 8 is to detect an input of an operating instruction and is to operate the grid-connected inverter 2 when the value of the generated voltage of the solar cell 1 is equal to or more than a specified value. When no operating instruction is inputted or when the value of the generated voltage of the solar cell 1, detected by a voltage detector (not shown), is less than the specified value, the control circuit 8 is to stop the operation of the grid-connected inverter 2.
The boost chopper 4 is formed of an inductor 41, a switching device 42 such as an IGBT, a diode 43 and a capacitor 44. Specifically, the inductor 41 is connected to the positive electrode side of the solar cell 1, to the corrector of the switching device 42, and to the anode side of the diode 43. Moreover, the emitter side of the switching device 42 is connected to the negative electrode side of the solar cell 1, and the cathode side of the diode 43 is connected to the emitter side of the switching device 42 through the capacitor 44. The output voltage of the boost chopper 4 is detected by the voltage detector (not shown), the detection signal of which is inputted to the control circuit 8. In the control circuit 8, processing of the inputted detection signal is carried out to determine the duty ratio of a pulse signal to be outputted. With the pulse signal having the determined duty ratio given to the gate of the switching device 42, the output voltage of the boost chopper 4 is controlled to a specified voltage.
The inverter circuit 5 is formed with the switching devices 51 to 54 such as IGBTs arranged in a full bridge connection. Each of the switching devices 51 to 54 is subjected to switching according to a PWM control carried out by the control circuit 8 to convert the DC electric power, outputted from the boost chopper 4, to AC electric power. The filter circuit 6 is formed of inductors 61 and 62 and a capacitor 63. Specifically, one end of the inductor 61 is connected to the connection point of the series connected switching devices 52 and 54 in the inverter circuit 5. One end of the inductor 62 is connected to the connection point of the series connected switching devices 51 and 53 in the inverter circuit 5. Moreover, the capacitor 63 is connected between the other ends of the inductors 61 and 62. With such a configuration, ripple components with a PWM carrier frequency are eliminated to make the output voltage of the inverter circuit 5 smoothed to have a sinusoidal waveform and outputted to the commercial power system 3.
The contactor 7 is connected between the filter circuit 6 and the commercial power system 3 to perform interconnection or disconnect between the grid-connected inverter 2 and the commercial power system 3 by a control signal outputted from the control circuit 8.
A current detector 9 detects an output current smoothed through the filter circuit 6 while the inverter circuit 5 is in operation. The detection signal of the detected output current is transmitted to the control circuit 8 and used for carrying out control so that the output current becomes in phase with the voltage of the commercial power system 3 detected by a voltage detector 10. Moreover, the current detector 9 is also used for detecting the flow of an overcurrent due to a failure of any one of the switching devices 51 to 54 in the inverter circuit 5. When an overcurrent is detected, the control circuit 8 decides that the grid-connected inverter 2 is in an abnormal condition to carry out actions such as a stop of the operation of the inverter circuit 5, a disconnect of the contactor 7, and a stop of the operation of the boost chopper 4 to stop the operation of the grid-connected inverter 2.
The voltage detector 10 detects the voltage of the commercial power system 3. The detection signal of the detected voltage is transmitted to the control circuit 8, which makes a decision as to whether or not each of the voltage and frequency of the commercial power system 3 is within a correct range required by the operational specification. When the detected voltage is out of the correct range, the control circuit 8 carries out operations such as stopping of the operation of the inverter circuit 5, disconnect of the contactor 7 and stopping of the operation of the boost chopper 4 to stop the operation of the grid-connected inverter 2.
If a disconnect signal is transmitted to the contactor 7 with the inverter circuit 5 stopped, the control circuit 8 decides from the voltage detected by the voltage detector 10 and the current detected by the current detector 9, that a reactive current flows in the filter circuit 6, and also decides that the contactor 7 is in an abnormal condition. If an interconnection signal is transmitted to the contactor 7 with the inverter circuit 5 stopped, the control circuit 8 decides that no reactive current flows in the filter circuit 6, and also makes a decision that the contactor 7 is in an abnormal condition.
FIG. 7 is a block diagram showing a solar cell power generation system as a second example of a grid connection system including the related grid-connected inverter disclosed in JP-A-2004-187362 in which an inverter current is detected.
A control circuit 8 makes the operations of a DC voltage conversion circuit 4 and an inverter circuit 5 stopped. Along with this, with a contactor 7 being opened immediately before starting an interconnection operation, the control circuit 8 takes in the results of detection by a voltage detector 13 to compare the value of the terminal voltage detected by the voltage detector 13 with a specified threshold value. When the value of the terminal voltage detected by the voltage detector 13 is higher than the specified threshold value despite the absence of an input signal for closing the contactor 7, the control circuit 8 decides that an abnormality such as contact adhesion occurs in the contactor 7. The control circuit 8, deciding that an abnormality occurs in the contactor 7, brings the DC voltage conversion circuit 4 and the inverter circuit 5 to a stopped state to discontinue the interconnection operation.
However, in the related grid-connected inverter 2 according to the above explained JP-A-2008-35655, with reference to FIG. 6, an overcurrent (a current equal to or more than 45 A, for example) is produced due to a failure such as a failure of any one of the switching devices 51 to 54. This requires the current detector 9 to be selected as one capable of detecting a current of a value exceeding the value of the overcurrent (50 A to 60 A, for example). Moreover, when the inverter circuit 5 is stopped and the contactor 7 is closed, the value of the reactive current flowing in the filter circuit 6 is on the order of 1.13 A, for example, which is a very small value since it is on the order of 2% of the value of a current that can be detected by the current detector 9. Thus, when the control circuit 8 is making a decision of the presence or absence of an abnormality in the contactor 7 on the basis of a minute current flowing in the capacitor 63 in the filter circuit 6, there was difficulty in precisely setting a threshold value for this purpose. This might cause the control circuit 8 to make an erroneous decision that the contactor 7 was abnormal.
Moreover, in the related grid-connected inverter 2 according to JP-A-2004-187362, with reference to FIG. 7, the voltage detector 13 was necessary for detecting the output voltage of the filter circuit 6 for making a decision of the presence or absence of an abnormality in the contactor 7. This caused a problem of required increases in cost and dimensions of the grid-connected inverter 2.
The invention was made to solve problems that arose in the related grid-connected inverters by actualizing a grid-connected inverter which can be made small in size at low cost, with a capability to reliably detect abnormalities in a contactor.